feat: recovering preHandleMetadata failure from sniffing (#769)

component/sniffer/dispatcher.go

@@ -35,7 +35,8 @@ type SnifferDispatcher struct {
 	parsePureIp     bool
 }
 
-func (sd *SnifferDispatcher) TCPSniff(conn *N.BufferedConn, metadata *C.Metadata) {
+// TCPSniff returns true if the connection is sniffed to have a domain
+func (sd *SnifferDispatcher) TCPSniff(conn *N.BufferedConn, metadata *C.Metadata) bool {
 	if (metadata.Host == "" && sd.parsePureIp) || sd.forceDomain.Has(metadata.Host) || (metadata.DNSMode == C.DNSMapping && sd.forceDnsMapping) {
 		inWhitelist := false
 		overrideDest := false
@@ -50,47 +51,50 @@ func (sd *SnifferDispatcher) TCPSniff(conn *N.BufferedConn, metadata *C.Metadata
 		}
 
 		if !inWhitelist {
-			return
+			return false
 		}
 
 		sd.rwMux.RLock()
 		dst := fmt.Sprintf("%s:%d", metadata.DstIP, metadata.DstPort)
 		if count, ok := sd.skipList.Get(dst); ok && count > 5 {
 			log.Debugln("[Sniffer] Skip sniffing[%s] due to multiple failures", dst)
 			defer sd.rwMux.RUnlock()
-			return
+			return false
 		}
 		sd.rwMux.RUnlock()
 
 		if host, err := sd.sniffDomain(conn, metadata); err != nil {
 			sd.cacheSniffFailed(metadata)
 			log.Debugln("[Sniffer] All sniffing sniff failed with from [%s:%d] to [%s:%d]", metadata.SrcIP, metadata.SrcPort, metadata.String(), metadata.DstPort)
-			return
+			return false
 		} else {
 			if sd.skipSNI.Has(host) {
 				log.Debugln("[Sniffer] Skip sni[%s]", host)
-				return
+				return false
 			}
 
 			sd.rwMux.RLock()
 			sd.skipList.Delete(dst)
 			sd.rwMux.RUnlock()
 
 			sd.replaceDomain(metadata, host, overrideDest)
+			return true
 		}
 	}
+	return false
 }
 
 func (sd *SnifferDispatcher) replaceDomain(metadata *C.Metadata, host string, overrideDest bool) {
+	// show log early, since the following code may mutate `metadata.Host`
+	log.Debugln("[Sniffer] Sniff TCP [%s]-->[%s] success, replace domain [%s]-->[%s]",
+		metadata.SourceDetail(),
+		metadata.RemoteAddress(),
+		metadata.Host, host)
 	metadata.SniffHost = host
 	if overrideDest {
 		metadata.Host = host
 	}
 	metadata.DNSMode = C.DNSNormal
-	log.Debugln("[Sniffer] Sniff TCP [%s]-->[%s] success, replace domain [%s]-->[%s]",
-		metadata.SourceDetail(),
-		metadata.RemoteAddress(),
-		metadata.Host, host)
 }
 
 func (sd *SnifferDispatcher) Enable() bool {

tunnel/tunnel.go

@@ -419,15 +419,28 @@ func handleTCPConn(connCtx C.ConnContext) {
 		return
 	}
 
+	preHandleFailed := false
 	if err := preHandleMetadata(metadata); err != nil {
 		log.Debugln("[Metadata PreHandle] error: %s", err)
-		return
+		preHandleFailed = true
 	}
 
 	conn := connCtx.Conn()
 	conn.ResetPeeked() // reset before sniffer
 	if sniffer.Dispatcher.Enable() && sniffingEnable {
-		sniffer.Dispatcher.TCPSniff(conn, metadata)
+		// Try to sniff a domain when `preHandleMetadata` failed, this is usually
+		// caused by a "Fake DNS record missing" error when enhanced-mode is fake-ip.
+		if sniffer.Dispatcher.TCPSniff(conn, metadata) {
+			// we now have a domain name
+			preHandleFailed = false
+		}
+	}
+
+	// If both trials have failed, we can do nothing but give up
+	if preHandleFailed {
+		log.Debugln("[Metadata PreHandle] failed to sniff a domain for connection %s --> %s, give up",
+			metadata.SourceDetail(), metadata.RemoteAddress())
+		return
 	}
 
 	peekMutex := sync.Mutex{}